Ionizing radiation cell survival curve parameters are generally discussed in the absence of considerations of the radiation chemical-chemical events which follow the deposition of energy in the cell. We have presented three possible molecular mechanisms to explain shouldered survival curves (a) the saturation of a repair process in conjunction with the progression of reparable to non-reparable damage (b) radiation induced oxidation of natural chemical repair agents -- sulfhydryl compounds and (c) binding of repair enzymes to radiation damaged sites in DNA. These hypotheses will be pursued; the dose of ionizing radiation which saturates repair of single strand breaks will be determined using Kohns alkaline elution technique. Sulfhydryl compounds will be assayed in cells before and after ionizing radiation using a sensitive fluorimetric assay. DNA polymerase activity will be assayed in cell extracts following exposure of the intact cell to ionizing radiation. Attempts will be made to interpret data from the above experiments (carried out in the presence and absence of radiosensitizers) to cell survival.